1. Field of the Invention
This invention relates to inverse planar transistors.
2. Description of the Prior Art
Integrated circuits in which transistors are operated inversely have already been discussed. In an inversely operated transistor, in contrast to a transistor constructed in the conventional planar technique, the emitter zone is not located on the surface of the semiconductor body or in an epitaxial layer deposited onto a semiconductor substrate, but is arranged in the semiconductor body itself, i.e., beneath the epitaxially deposited layer or in it.
These integrated circuits are referred to as MTL or I.sup.2 L circuits (merged-transistor-logic or integrated-injection logic).
In an inversely operated transistor, the emitter zone generally consists of a highly doped, so-called "buried layer," which is diffused or implanted into the semiconductor substrate prior to the application of the epitaxially deposited layer; here the part of the epitaxially deposited layer which adjoins the base zone must also be considered as part of the emitter zone.
Usually the collector zone of an inversely operated transistor is produced by the diffusion of a specific doping material, for example, phosphorus or arsenic for npn-transistors. In the planar technique, this diffusion represents the last high-temperature step and is particularly critical since generally this last diffusion step cannot be prevented from influencing the doping conditions in the base zone of the transistor. In the event that a base zone is doped with boron, this influence can, for example, lead to a so-called "dip-effect," which in turn leads to an expansion of the base zone beneath the collector zone when phosphorus is provided as doping material for the collector zone. When the base zone is doped with boron while arsenic is used for the diffusion into the collector zone, a so-called "retardation" or indentation can occur in the base zone beneath the collector zone.
As the base width, i.e., the distance or the difference in thickness between the base penetration depth and the collector penetration depth has a decisive influence on the electric parameters, such as, for example, the cut-off frequency of a transistor, generally it is undesirable that the doping conditions in the base zone should be influenced during the production of the collector zone.